1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to an apparatus and method for enabling a user equipment (UE) to receive and demodulate signals transmitted from a Node-B using a transmit diversity method.
2. Description of the Related Art
As mobile communication systems are rapidly developed and an amount of service data to be provided by the mobile communication systems are rapidly increased, a third generation mobile communication system has been developed to transmit a large amount of data at a high rate. In terms of the third generation mobile communication system, a wideband-code division multiple access (W-CDMA) system operates in an asynchronous mode between Node-Bs in Europe and a code division multiple access-2000 (CDMA-2000) system is standardized as a radio access standard in North America.
The mobile communication system typically configures one Node-B and a plurality of user equipments (UEs) so that the Node-B and the UEs can communicate with one another. However, when data is transmitted at a high rate in the mobile communication system, a received signal can be distorted by a fading phenomenon on a radio channel. Where the received signal distorted by the fading phenomenon is not compensated for at the time of demodulating data of the received signal since the fading phenomenon can reduce the amplitude of the received signal by from several decibels (dB) to several tens of decibels (dB), transmission data of a transmitter can be erroneous and hence the quality of mobile communication service can be degraded. Thus, the fading phenomenon must be overcome so that the mobile communication system can transmit data at a high rate without degrading quality of service (QoS). To overcome the fading phenomenon, various types of diversity schemes are used.
Conventionally, the CDMA system uses a Rake receiver for performing diversity reception using the delay spread of a channel signal. The Rake receiver receives multipath signals in a reception diversity manner. One signal path is assigned to each finger of the Rake receiver so that the Rake receiver can perform a demodulation operation. However, there is a drawback in that the Rake receiver based on diversity technology using the above-described delay spread does not operate where a value of the delay spread is smaller than a set value. Furthermore, a time-diversity scheme using interleaving and coding technologies is typically used in a Doppler spread channel. However, there is a drawback in that it is difficult for the time-diversity scheme to be used in a low-speed Doppler spread channel.
To overcome the fading phenomenon, a space-diversity scheme is used in a channel with low delay spread such as an indoor channel and a channel with low-speed Doppler spread such as a pedestrian channel. The space-diversity scheme uses two or more transmitting and receiving antennas. In other words, where a level of a signal transmitted through one antenna is reduced because of the fading phenomenon, the space-diversity scheme allows a demodulation operation to be performed for signals transmitted through the remaining antenna or antennas.
The space-diversity scheme includes a receive antenna diversity method using receive diversity antennas and a transmit diversity method using transmit diversity antennas. When the receive antenna diversity method is applied to a UE, a plurality of antennas must be installed in the UE and hence the cost and size of the UE increases. Thus, it is recommended that the transmit diversity method be used in a Node-B in which a plurality of antennas are installed.
According to the transmit diversity method, a transmitter transmits signals using multiple antennas, and a receiver receives, demodulates and combines respective antenna signals, such that a fading channel can be compensated for. Typically, the above-described W-CDMA system uses two antennas provided in the Node-B to implement the transmit diversity method.
The transmit diversity method indicates an algorithm for receiving downlink signals and acquiring a diversity gain. The transmit diversity method includes an open-loop transmit diversity method and a closed-loop transmit diversity method. Furthermore, the open-loop transmit diversity method includes a time-switched transmit diversity (TSTD) scheme and a space-time transmit diversity (STTD) scheme. The closed-loop transmit diversity method includes a transmit antenna array (TxAA) scheme
When a Node-B encodes information bits and transmits the encoded information bits through diversity antennas according to the open-loop transmit diversity method, a UE receives and decodes signals from the Node-B, such that a diversity gain can be acquired.
According to the closed-loop transmit diversity method, (1) the UE predicts channel environments that signals transmitted through transmitting antennas provided in the Node-B undergo, and calculates prediction values, (2) the UE calculates weights of the Node-B's antennas in which the maximum electric power for received signals can be generated using the calculated prediction values and transmits the calculated weights to the Node-B through an uplink, and (3) the Node-B receives weight signals from the UE and adjusts weights of the antennas. Here, the Node-B transmits pilot signals discriminated on an antenna-by-antenna basis to measure a channel of the UE, and the UE measures the channel through the pilot signals discriminated on the antenna-by-antenna basis and searches for an optimum weight as measured channel information.
The closed-loop transmit diversity method and the open-loop transmit diversity method will now be described in detail with reference to FIGS. 1 and 2.
FIG. 1 is a block diagram illustrating a transmitter using the TxAA scheme for implementing the closed-loop transmit diversity method.
A dedicated physical control channel (DPCCH) configured by a plurality of control signals and a dedicated physical data channel (DPDCH) configured by data signals are multiplexed to form a dedicated physical channel (DPCH). The DPCCH and DPDCH are typically multiplexed in a downlink using a time multiplexing operation.
A DPCH signal is scrambled with a scrambling code by a multiplier 104. On the other hand, the scrambled DPCH signal is multiplied by predetermined weights W1 and W2 by multipliers 106 and 108 so that the scrambled DPCH signal is applied to the closed-loop transmit diversity method. Output signals of the multipliers 106 and 108 are transmitted through the first antenna 114 and the second antenna 116, respectively.
Closed-loop transmit diversity methods are classified into schemes according to weight application types. In other words, the closed-loop transmit diversity methods include a closed-loop transmit diversity mode 1 scheme for deciding weights by taking into account a phase difference between signals received from the two antennas, and a closed-loop transmit diversity mode 2 for determining weights by taking into account a level difference between the signals received from the two antennas along with the phase difference.
The signals are multiplexed with specific pilots signals Common Pilot Channel 1(CPICH1) and Common Pilot Channel 2 (CPICH2) set on the antenna-by-antenna basis by multiplexers 110 and 112 so that a receiver can distinguish between the signals transmitted from the first and second antennas.
The UE's receiver receives the signals transmitted through the antennas, and the UE measures channel states for the antennas through the pilot signals and determines weights for the antennas so that reception signal power corresponding to a sum of the two antenna signals can be maximized. Weight information is defined as a set of several preset weights. The receiver selects one weight from the set of several preset weights so that reception electric power can be maximized. If information of the selected weight is transmitted through a feedback information (FBI) field contained in an uplink DPCCH message, an FBI message determinator 118 of the transmitter analyzes the FBI field received from the receiver, and a weight generator 120 generates the weights W1 and W2 on the antenna-by-antenna basis so that the weights W1 and W2 can be multiplied by the DPCH to be transmitted, respectively.
A channel encoding operation based on the STTD scheme of the open-loop transmit diversity method will now be described with reference to FIG. 2. FIG. 2 is a schematic diagram illustrating the channel encoding operation through an STTD encoder.
Symbols are sequentially input into the STTD encoder (not shown) according to transmit diversity encoding times used in the transmit diversity method. Then, the STTD encoder encodes the input symbols according to the STTD scheme, and transmits the encoded symbols to the two transmitting antennas. For example, when symbols S1 and S2 are sequentially input into the STTD encoder during transmit diversity encoding times T1 and T2, the STTD encoder encodes the sequentially input symbols S1 and S2, transmits a symbol signal S1S2 through the first antenna, and transmits a symbol signal −S2*S1* through the second antenna.
Referring to FIG. 2, it is assumed that the symbols S1 and S2 are sequentially input according to the transmit diversity encoding times are configured by channel information bits 202 indicating b0b1 and b2b3. The channel information bits 202 indicating b0b1and b2b3 corresponding to the symbols S1 and S2 are input into the STTD encoder. The STTD encoder encodes the channel information bits 202 indicating b0b1b2b3, outputs channel information bits 204 indicating b0b1b2b3 (S1S2) to the first antenna, and outputs channel information bits 206 indicating −b2b3b0−b1, (−S2*S1*) to the second antenna.
On the other hand, a channel structure of the W-CDMA system includes a physical channel, a transport channel and a logical channel. The physical channel has a structure of downlink and uplink physical channels according to information and data transmission directions. Furthermore, the downlink physical channels are classified into a physical downlink shared channel (PDSCH) and a downlink dedicated physical channel (DPCH).
When a signal is sent through the DPCH, the above-described STTD scheme is used as the open-loop transmit diversity method (according to a universal mobile telecommunications system (UMTS) standard TS 25.211). Channels using the STTD scheme include a primary-common control physical channel (P-CCPCH), a secondary-common control physical channel (S-CCPCH), a synchronization channel (SCH), a page indication channel (PICH), an acquisition indication channel (AICH), a PDSCH, etc.
Transmit diversity methods used in the physical channels are shown in the following Table 1.
TABLE 1Open-loopPhysical channeltransmit diversityClosed-looptypeTSTDSTTDtransmit diversityP-CCPCHXOXSCHOXXS-CCPCHXOXDPCHXOOPICHXOXPDSCHXOOAICHXOXCSICHXOXAP-AICHXOXCD/CA-ICHXOXDL-DPCCH forXOOCPCH
At this time, requirements in applying the transmit diversity method to the physical channels are as follows.
1) The STTD scheme and the closed-loop transmit diversity method cannot be simultaneously applied to the same physical channel.
2) Where the transmit diversity method is applied to any downlink, it must be always applied to the P-CCPCH and SCH.
3) The PDSCH and the DPCH corresponding to the PDSCH must use the same transmit diversity method.
Conventionally, the CDMA mobile communication system uses the above-described Rake receiver. Where the downlink DPCHs are received from two or more Node-Bs in soft handover, a finger is assigned to one of multiple paths. That is, multipath signals from the Node-Bs are assigned to fingers of the Rake receiver, and the fingers receive the multipath signals.
The above-described method enables a transmit diversity signal processor provided in each finger stage to process each multipath signal according to a transmit diversity method for each Node-B.
FIG. 3 is a block diagram illustrating the Rake receiver equipped with transmit diversity processors coupled to fingers. As described above, the receiver of the UE includes a plurality of fingers so that signals on a path-by-path basis are demodulated to compensate for multipath fading. Furthermore, where the transmitter uses space diversity through two antennas, the receiver demodulates two types of signals received from the two antennas on a finger-by-finger basis. In other words, demodulation signals associated with the first and second antennas 302 and 304 are generated from the first finger and demodulation signals associated with the first and second antennas 306 and 308 are generated from the second finger.
A W-CDMA-based UMTS combines DPCHs transmitted from a plurality of Node-Bs in soft handover. At this time, the use of a transmit diversity method between the Node-Bs transmitting the DPCHs capable of being simultaneously received by the receiving side must obey the following rules according to a standard.
1) Upon transmitting the DPCHs to a desired UE, the Node-Bs use one transmit diversity method. In other words, the Node-Bs cannot simultaneously use the open-loop transmit diversity method and the closed-loop transmit diversity method.
2) When all Node-Bs currently performing a transmission operation use no transmit diversity method, the use of a transmit diversity method in a Node-B desiring to transmit a new DPCH is not affected by other Node-Bs.
3) When at least one of the Node-Bs currently performing a transmission operation transmits a DPCH using the open-loop transmit diversity method, a new Node-B can transmit a DPCH using the open-loop transmit diversity method or without using the open-loop transmit diversity method.
4) When at least one of the Node-Bs currently performing a transmission operation transmits a DPCH using the closed-loop transmit diversity mode 1 , a new Node-B can transmit a DPCH using the closed-loop transmit diversity mode 1 or without using the closed-loop transmit diversity mode 1.
5) When at least one of the Node-Bs currently performing a transmission operation transmits a DPCH using the closed-loop transmit diversity mode 2, a new Node-B can transmit a DPCH using the closed-loop transmit diversity mode 2 or without using the closed-loop transmit diversity mode 2.
In other words, where the downlink DPCHs are received from two or more Node-Bs in soft handover, a transmit diversity method to be used by each Node-B is not affected by that used by other Node-Bs. On the other hand, when the transmit diversity method is used, the open-loop transmit diversity method and the closed-loop transmit diversity method cannot be simultaneously used. When the closed-loop transmit diversity method is used, the closed-loop transmit diversity modes 1 and 2 cannot be simultaneously used. Consequently, only one of the transmit diversity methods must be selected and used.
In one method for satisfying the above-described standard, a receiver for processing the diversity can assign one or more fingers for a plurality of Node-Bs. For example, when four fingers are provided in the receiver, two fingers can be assigned to receive signals from the first Node-B, and other two fingers can be assigned to receiver signals from the second Node-B. In other words, as a plurality of fingers are assigned in relation to one Node-B, a demodulation operation can be performed using diversity according to multiple paths. Furthermore, since the demodulation operation can be performed on a finger-by-finger basis and on a transmitting antenna-by-antenna basis, the demodulation operation can be performed on the basis of transmit diversity.
Referring to FIG. 3, the demodulation operation must be performed by taking into account a transmit diversity method of each Node-B when it is assumed that a plurality of fingers are assigned for a plurality of Node-Bs. For example, where the first and second fingers are assigned for the demodulation operation associated with the first Node-B and the first Node-B uses a space-time transmit diversity (STTD) scheme for open-loop transmit diversity as the transmit diversity method, signals discriminated by the first and second fingers must be demodulated by the first and second transmit diversity signal processors 322 and 324 provided in the transmit diversity signal processor 320 on the basis of the STTD scheme.
Furthermore, where the third and fourth fingers are assigned for the demodulation operation associated with the second Node-B and the second Node-B performs a transmission operation without using transmit diversity, signals detected by the third and fourth fingers are sent to a combiner 330 without being applied to the transmit diversity signal processing operations of the third and fourth transmit diversity signal processors (not shown) provided in the transmit diversity signal processor 320.
The transmit diversity signal processors 322 to 326 perform transmit diversity signal processing operations for multipath signals obtained from the fingers assigned on a Node-B basis, signals corresponding to a result of the transmit diversity signal processing operations are combined by the combiner 330, and the combined signals are finally output.
Transmit diversity signals can be independently processed as shown in FIG. 3. However, there is a problem in that the complexity of hardware increases as the number of fingers increases. Furthermore, since demodulation operations based on all transmit diversity methods must be able to be performed through the fingers according to various types of transmit diversity methods, a structure of the receiver becomes complex.
Thus, there is another problem in that a structure for performing a transmit diversity signal processing operation in the Rake receiver equipped with the large number of fingers is inefficient.